A boiler for making super heated steam is described in U.S. Pat. No. 3,867,907. In this apparatus a hot synthesis gas flows through tubular pipes, which are located in a water bath located at the lower end of a vertically oriented vessel. In said lower end saturated steam is generated. In the upper end of the vessel a conduit having a larger diameter than the tubular pipes surrounds said tubular pipes thereby defining an annular space around said pipes. The lower end of said annular space is open to receive saturated steam, which flows co-current with the hot syngas to the upper end of the vessel. At said upper end super heated steam and cooled synthesis gas are separately discharged from said vessel. The publication is especially directed to a protective cup around the inlet opening for saturated steam of the annular space.
A disadvantage of said design is that liquid water may enter the annular space, which will negatively affect the production of super heated steam. Another disadvantage is that local overheating at the inlet of the annular space may occur which will give rise to mechanical failure of the pipes. Because boilers of this type are designed to operate for years without failure any possible overheating due to the design should be avoided.